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876                            Appendix II

4.  Consumer's motor to be simple and efficient.

5.  Facility for adaptation to numerous uses.

Only two great natural sources of power have been much used
up to the present, gravity and heat, the former being the water
power of streams, and the latter the latent energy of coal or
petroleum. The distributors have been electricity, wire rope,
compressed air, hydraulics, or coal gas ; of which probably com-
pressed air meets the above requirements most perfectly. Inter-
mittent use means the necessity for storage if a part of the plant
is not to lie idle at times, and electric accumulators are very
costly for large stations, but hydraulic power may be efficiently
stored. A low-pressure water power is used in Switzerland, drawn
from reservoirs 400 ft. high kept filled by turbines, and the
storage of heat has been accomplished by admitting surplus
boiler steam to a large vessel of water under high pressure, the
liquid thus heated being converted into steam when needed, at a
small reduction of pressure.


P. s#j. Conduction of Heat through Plates.  The
boiler furnace having a temperature of about 1500, and the steam
of 300, it may be taken that there is a (skin' drop of 500 on
each side, and another drop of 300 in passing through the plate.

P. j#f. Le Chatelier's Pyrometer is undoubtedly the
finest modern apparatus for measuring high temperatures, and, as
improved by Roberts-Austen, is shewn in Fig. 836. A is a thermo-
electric couple of two wires, twisted together, one of platinum and
the other of platinum alloyed with 10% of rhodium. Being pro-
tected with refractory material, they are placed in a source of heat,
and connected to the mirror galvanometer B, within the camera
c. A ray of light from an oxy-hydrogen burner D is deflected and
I N                         focussed on to the mirror E, from which it returns to a photo-

11|                         graphic plate F.    Now the horizontal movement of the light ray at

ff will indicate temperature: but a better way is to cause the plate
F to move vertically by clockwork while the heat changes occur at